E-type cyclins (E1 and E2) are components of the core cell cycle machinery. Overexpression of E-cyclins has been documented in a large number of human cancers. For this reason, E-cyclins might represent therapeutic targets in cancer patients. However, E-cyclins were thought to be required for proliferation of all cell types, thereby precluding their targeting in cancer therapy. We decided to rigorously test this notion by generating cyclin E-null mice. Surprisingly, we found that cyclin E-null embryos survived till mid-gestation. Cyclin E-null fibroblasts proliferated normally under conditions of continuous cell cycling, but they were unable to re-enter cell cycle from G0. We also found that E-null cells showed reduced sensitivity to oncogenic transformation. It has been assumed that E-cyclins function as activating subunits of cyclin-dependent kinases, mostly CDK2. However, during the last funding period we demonstrated that a kinase-dead cyclin E1 mutant, which is unable to form active complexes with CDKs, restored the ability of cyclin E-null cells to re-enter the cell cycle, and also restored their oncogene-sensitivity. We also generated a conditional cyclin E knockout mouse strain, and we found that ubiquitous ablation of cyclin E in adult animals had no major impact on their health. On the other hand, we found that even reducing the levels of E-cyclins (E1-/-E2 or E1E2-/- animals) rendered mice less sensitive to Myc-driven breast cancers. These observations suggest that ablation of cyclin E in tumor-bearing organisms might specifically cripple the proliferation of cancer cells, while sparing other tissues. Lastly, during the last funding period we developed a novel knock-in strain of mice expressing tandemly (FLAG- and HA-) tagged cyclin E1. Using this strain, we employed sequential immunoaffinity purifications followed by mass spectrometry to determine the identity of cyclin E1 binding partners in mouse organs in vivo. We observed several novel tissue-specific interactors. This work raises several fundamental questions: (1) we found that the function of cyclin E in G0 exit was kinase-independent. Does cyclin E also play a kinase-independent function in other processes during mouse development? Likewise, does cyclin E play a kinase-independent function in cancer formation in vivo, and - if it does - what is its exact molecular basis? We will address these questions in Aim 1. (2) We observed that mice deficient in cyclin E showed decreased sensitivity to Myc-driven breast cancers. From a clinical standpoint it is very important to determine whether an acute cyclin E shutdown in tumor-bearing mice would halt the oncogenic proliferation of tumor cells. We will address this issue in Aim 2. (3) In the last funding period, we identified novel, tissue-specific interactors of cyclin E. We will follow up on these observations at a mechanistic level in Aim 3. The Aims are: 1. To study the contribution and the molecular basis of cyclin E kinase-independent function in mouse development and in tumorigenesis; 2. To test the requirement for cyclin E function to maintain the tumorigenic potential of breast cancer cells using conditional cyclin E knockout mice; 3. Analyses of novel tissue-specific cyclin E functions in vivo.